In recent years, in order to increase the visibility of image display devices, the gap between an image display panel such as a liquid crystal display (LCD), a plasma display (PDP) or an electroluminescence display (ELD), and a protective panel or a touch panel member disposed on the front side (viewing side) thereof, is filled with an adhesive sheet, a liquid bonding agent, or the like, to carry out suppression of reflection of incoming light or outgoing light from a displayed image at the air layer interface.
As a method that uses an adhesive for filling such a gap between constitutive members for display device, a method is known, whereby a liquid adhesive resin composition containing a UV-curable resin is filled in the gap and then illuminated with UV light to let it cure. However, such a method bears the problem that the operation when the liquid is filled being cumbersome, productivity is poor, and moreover, at locations that UV light has difficulty in reaching such as portions masked by a printed masking layer, curing the adhesive is difficult, and obtaining a stable quality is difficult.
In addition, the method of filling the gap between constitutive members for display device using an adhesive sheet is also known. For instance, as a transparent adhesive sheet that can be used suitably for laminating a transparent panel such as a protective panel or a touch panel to an image display panel, a transparent adhesive sheet is described in Patent Document 1, which is a adhesive sheet having at least one or more each of a first adhesive layer and a second adhesive layer having different viscoelastic behaviors and is provided with a constitution in which these layers are layered integrally, the value of the dynamic shear storage modulus G′ as measured with a temperature distribution at 1 Hz frequency being within a specific range.
A transparent double-sided adhesive sheet is described in Patent Document 2, which is a transparent double-sided adhesive sheet having a middle resin layer (A) and pressure-sensitive adhesive layers (B) as the front and back side layers, each layer being respectively a layer with one or more species of (meth)acrylic acid ester series (co)polymer as the base resin, the storage shear modulus (G′(A)) of the middle resin layer (A) at 1 Hz frequency in the 0° C. to 100° C. temperature range being higher than that of the pressure-sensitive adhesive layers (B), and, the indentation hardness (Asker C2 hardness) of the entirety of the sheet being 10 to 80.
In addition, as a thin (for instance, 30 to 50 μm-thick) adhesive sheet that is applicable onto a surface having a step or a protrusion, a UV-crosslinking adhesive sheet is described in Patent Document 3, which is a UV-crosslinking adhesive sheet comprising a (meth)acrylic copolymer of a monomer containing a (meth)acrylic acid ester having a UV-crosslinking site, the store elastic modulus of the adhesive sheet before UV-crosslinking being 5.0×104 Pa or greater but 1.0×106 Pa or lower at 30° C. and 1 Hz and 5.0×104 Pa or lower at 80° C. and 1 Hz, and further, the store elastic modulus of the adhesive sheet after UV-crosslinking being 1.0×103 Pa or greater at 130° C. and 1 Hz.